It seems you have too many search requests queued up and your nodes can't keep up with the pace you're setting.

You can read the exception message like this: Your search thread pool has 13 threads (pool size = 13) and all are busy (active threads = 13). By default, when all threads are active, all search requests coming in the meantime are queued waiting for a thread to be available to handle them. In your case, that queue is full (queued tasks = 1000). This exception is caused by the 1014th search request coming in (1000 waiting + 13 active being processed + this one). Increasing the queue size might work, but it's only postponing the problem.

This means that your 8 CPUs/cores nodes (the pool size is determined by the following equation: int((# of available_processors * 3) / 2) + 1) are not able to keep up with the load your client app is throwing at it. You might need to scale out (i.e. add nodes) in order to handle a higher capacity.